Monter Seminar JAM Inquiries

Event Information

Monster Seminar JAM - What drives the abundance of top predators in marine ecosystems? An integrated case study of seabird and fur seal population dynamics in the eastern Bering Sea

Presentator(s):

Dr. Andrew Trites, Professor, Dept. of Zoology, University of British Columbia

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Abstract

We sought to determine why northern fur seals, thick-billed murres and black-legged kittiwakes are thriving on Bogoslof Island (southern Bering Sea) and declining on St. Paul Island (Pribilof Islands central Bering Sea). To test whether the declines were associated with hypothesized reductions in prey caused by fishing, our team of seabird biologists, fishery scientists and marine mammal experts assessed the abundance and distribution of prey around each breeding colony (JulAug 2009), and concurrently determined the diets, stress levels, and foraging locations of the murres, kittiwakes and fur seals. The birds and seals from the Pribilof Islands were found to travel further and for longer times to feed, and experienced higher stress levels compared to those from Bogoslof Island. Diets were also found to differ significantly between islands. Most notably, diets of fur seals and murres on St. Paul Island were dominated by juvenile walleye pollock (low energy content prey), whereas diets on Bogoslof were dominated by high energy mesopelagic species (squids for murres; myctophids for kittiwakes; squids and northern smoothtongue for seals). At-sea distributions of birds and mammals were relatively uniform across the Bering Sea, but distinct between breeding islands. Seal and seabird distributions corresponded with the broad scale distribution of widely dispersed shallow patches of preyand not with the total biomass or numerical abundance of their dominant prey species. Quality of diets (i.e., energy density) and their accessibility appear to drive reproductive success of birds and seals in the Bering Sea. Our comparative analysis of dietary information collected before the Pribilof population declines further suggests that recovery of northern fur seals, thick-billed murres, and black-legged kittiwakes to numbers present prior to the oceanic regime shift (1975-1976) will require another regime shift to increase the abundance and availability of energy-rich cephalopods and mesopelagic fishes.